So the quitting disease amongst frontbenchers has spread to the Tory party.

After Estelle Morris left government declaring she wasn't up to the job John Bercow has abandoned the shadow cabinet on a matter of principle.

It's all very disconcerting for those who believe the first law of politics is never quit until you are sacked.

Bercow went on principle

And Mr Bercow was most definitely not forced out - other than by what he clearly believes is Mr Duncan Smith's leadership style.

The excitable frontbencher - seen as a rising star who is far cleverer than his performances often suggest - thinks the vote over whether gay and unmarried couples should be allowed to adopt should be a matter of conscience.

Mr Duncan Smith, no doubt in a bid to shore up his creaking support from his party's right wing, thinks otherwise and has ordered his troops to back the family, as he sees it.

But this rebellion looks like spreading, and giving the newly high-profile Michael Portillo - who is defying the leader he insists he does not want to oust - a clear sign of who his potential storm troopers may be.

Mods and rockers

This row has clearly come at a particularly difficult time for the Conservative leader.

The public mutterings about leadership challenges continue, and have probably been encouraged by Mr Duncan Smith's tactics.

At the centre of the discontent is the battle between the modernising "mods" and the traditionalist "rockers".

Hague wanted broader appeal

Mr Bercow is a leading mod and his resignation sends out the clearest possible signal that he believes much of Mr Duncan Smith's talk about inclusion is not being carried through into practice.

It is precisely the same accusation that his predecessor William Hague - who started all the talk about inclusion - suffered.

You would think that would delight the rockers. And it has even been suggested that the adoption move was specifically designed to appease them.

But they show little sign of coming over to Mr Duncan Smith simply because of his opposition to adoption by gay and unmarried couples

Neither do they seem particularly impressed by the loss of a top mod from the shadow cabinet.

Wider revolt

And in the ensuing shambles, the Tory party has lost a frontbencher - and Mr Duncan Smith's attempt to look hard has backfired.

With the current level of experience and talent on the Tory frontbenches, Iain Duncan Smith can't afford to start shedding shadow ministers.

That is bad enough, but it also looks like he has a wider revolt on his hands over this issue.

The mods appear to be on the march and are using this issue as a rallying point.

It could yet turn into a significant challenge for the leader who needs such a row like he needs a hole in the head.

Mr Bercow, meanwhile, has emerged as a principled operator who will now be seen as a bit of a hero by the Tory left.

Should there be the much talked about coup against Mr Duncan Smith, and the mods take control, he will be in prime position for a top job.